Currently, in the semiconductor industry there is a great interest in the characterization of thin films. Integrated circuits are made up of a large number of thin films deposited onto a semiconductor substrate, such as silicon. The thin films include metals to make connections between the transistors making up the chip, and insulating films to provide insulation between the metal layers (see: S. A. Campbell, The Science and Engineering of Microelectronic Fabrication, Oxford University Press, (1996)). The metal films (interconnects) are typically arranged as a series of patterned layers. At the present time there may be 4 or 5 layers of interconnects. It is likely that as more complex integrated circuits are developed which will require a greater number of interconnections, the number of layers will increase. Metals of current interest include, for example, aluminum, cobalt, copper, titanium, and silicides. Insulating films include, for example, oxide glasses of various compositions and polymers.
In the production of integrated circuits it is essential that all aspects of the process be controlled as closely as possible. For metal films, it is desirable to measure properties such as the film thickness, the electrical resistivity, the grain size, the grain orientation, and the roughness of the surfaces of the film.
Currently available techniques for the determination of grain orientation include the following.
1) Electron Back-Scatter Diffraction. A sharply-focussed electron beam is directed onto the surface of the sample at an oblique angle. The back-scattered electrons diffracted from the atomic planes within the sample are detected. The intensity of these electrons form characteristic patterns, referred to as Kikuchi lines. From the angular positions of these lines, the crystallographic orientation of the atoms in the region of the sample 10 where the electron beam is incident can be determined. It is possible to scan the electron beam across the sample, and to determine how the crystallographic orientation varies from grain to grain. In this way the distribution of grain orientations can be obtained. This method has the disadvantage that a considerable amount of time is required to make a measurement. In addition, the sample 10 has to be placed into a high-vacuum chamber for the measurement to be made.
2) Transmission electron microscopy. In this technique the diffraction of high energy electrons passing through the sample is measured. The grain orientation can be determined from the diffraction pattern. To make this type of measurement, it is essential to reduce the thickness of the sample so that high energy electrons can be transmitted. Thus, the method suffers from the disadvantage that the sample is destroyed. A second disadvantage is that the preparation of the sample takes a considerable amount of time. A third disadvantage is that the sample has to be placed into a high-vacuum chamber for the measurement to be made.
3) X-Ray Diffraction: In this technique X-rays are directed onto the surface of the film, and the diffracted X-rays are detected to determine the grain orientation. This method cannot be used for rapid measurements of grain orientation.